The proposed research includes three objectives: 1) Study the effect of ecological changes due to formation of a man-made lake in a tropical forest on the natural history of arboviruses; 2) Test the hypothesis that endemic strains of VEE virus can mutate into epidemic strains by passage through abnormal hosts and/or vectors; 3) Investigate existence of transmission cycles of yellow fever other than mosquito-primate one. Investigations on the first objective are taking place on the projected Bayano hydroelectric dam 70 kms east of Panama City which will form a lake of 300 square km. A camp was established in the forest of a future island in the lake. Three years of base-line pre-flooding information has been gathered including microclimatic data, ecological studies on vectors and hosts and virological investigations on arboviruses detected in the area. Plans call to continue studies during formation and stabilization of the lake. Studies on VEE call for inoculation of early-passaged material of endemic VEE strains into herons and using five species of neotropical mosquitoes to keep cyclical transmission heron-mosquito-heron and observe possible changes in antigenic properties and pathogenicity that might result. Search for other mechanisms of transmission of YF virus will be done both in the field in the border area between Panama and Colombia and in the laboratory.